Since my OCZ powerstream 600 left the realm of earth I'm without any PSU which features a tweakable 3,3V line. No problem, I've had experience with modding ATX PSU's (Building a 300W Fan Controller from an ATX PSU: http://www.madshrimps.be/?action=gethowto&howtoID=77). I took one of the many elder PSU's I have around and began modifying its output voltage.
Good thing it had a on board trimmer so there was little searching involved, but I discovered that even though the 12V would rise as high as 13.5V, the 3.3V line never went higher then 3.35V. Even higher brought the PSU in lockdown mode. Hmm, but on the other hand I could easily bring down the 5V line as low as 3V, maybe that could be of use?

Well, so I came to the idea to have one PSU feeding all necessary power levels (+12V, +5V, +5V standby,...) except for the 3.3V line, therefore I would use the other ATX PSU which I just modified.

What I had to do is giving both PSU's the same reference 0V. I cut off the ATX connector from the tweakable PSU (slave PSU) and soldered the black wires to the same spot as the black wires inside the main PSU (master PSU). This way 0V is 0V in both PSU's, no floating volt lines here. I also desolderd all ~220V net line cables inside the slave PSU and made a pass-through from the master to the slave PSU so that I would need only one power cord for both PSU's. Next I cut of the green cable from the slave PSU and connected it to the green cable inside the master PSU's, this way both PSU's will power on at the same time. For easier testing purposes I've put an on/off switch in parrallel, its much alike the green wire PSU-on trick.

I also got rid of all the cables I wont ever use with the slave PSU, I will only be using the tweakable 5V line. To keep the colors matching I desoldered the orange 3.3V line cables and put them in place of the red 5V line cables, that's why you see orange cables leaving the PSU. These cables are routed inside the main PSU were I disconnected the original orange cables from the PCB, the original 3.3V line will then be unused. In order to have my 3.3V back at the 20-pins ATX plug I made a connection between the cables from the ATX connector from the master PSU and the orange cables coming from the slave PSU. End result: I have one PSU feeding normal voltages while the second PSU handles the 3.3V line voltage on its own. Via a potentiometer I can easily adjust output voltage of the 3.3V line up to nearly 5V!

A schematic:

Some pictures I took during testing;

Assembled, back side picture:

Assembled front side picture:

And off course a video where I regulate the 3.3V line voltage all the way up to its maximum. Well, I hardmodded the maximum, sure you can get more but what's the point?

The total unit is working very well on my Gigabyte GA-5SMM socket 7 board, though when using the ASUS P5A board the system does not boot. No idea why, the board only boots with one of the three units I have around here.

Got them PSU's back open, 5V=5V under load but 12V=11,5V under load, I'm going to add regulation so to have higher volts when the system is under heavy stress. Might maybe help getting the SIS530 based motherboard stable at 133MHz strap.